I have a party consisting of two PCs; a lv5 fighter (3 fighter, 1 rogue, 1 barbarian) , and a lv5 ranger (4 ranger, 1 cleric).
Focusing on the ranger for the purposes of this question. She doubles as our caster, and it's been working super well for us. However, we're feeling really hemmed in by the way prepared spells are managed in 5e. As a GM I don't like a) hounding her to pick spells after each long rest and b) limiting what they can do just because she didn't magically see into the future to realize Comprehend Languages would have been useful today.
I want to change how she prepares spells to allow her the roleplaying freedom to pick some spells at the time of casting, rather than needing to set them ahead of time. On the flip side I can see a lot of room for abuse so I'm considering offering one of the following two systems but would like someone to double check my logic (I'm a first-time GM and fairly unaware of what the later levels will bring.)
At this point her prepared spells work out like so;
She has access to eight spells total. Five are predetermined; three because she's a ranger and knows those spells, and two which are always prepared for her because of her cleric domain. This leaves three spells she must prepare at the beginning of each day.
I see two logical ways to resolve this.
Method 1
Of the three remaining prepared spells, one can be swapped at will at the cost of an action. The other two must be prepared ahead of time like normal.
IE, she can take a turn during combat to switch one spell for another spell of her choice (or prepare one if the slot was previously empty).
Needing to take an action to prepare the spell should be enough of a limiter to prevent abuse, and it gives her plenty of flexibility during combat. However, outside of combat it would logically follow that she can switch spells, at will, every 6 seconds. That seems pretty game breaking.
Method 2
The remaining prepared spells are empty until used. She can declare the use of a spell at any time for no penalty, but once the spell is declared she can't switch it again until the next long rest.
IE, over the course of a day she can select any three spells she likes to use but there's no option to change them once selected.
This feels more balanced between in-and-out of combat scenarios, but I can see it potentially getting out of control in later levels. I was thinking of restricting this down to just 1-2 "empty" prepared spells (the rest would be pre-prepared as usual) as a limitation if three is too broad.
TL;DR:
I want to give my player more flexibility in her prepared spells. Is one of these two methods more acceptably balanced than the other? Will it become unbalanced as she levels up?
Best Answer
While it's hard to judge balance, especially when it's not clear what you're balancing against, these two house-rules both have a pretty effective point of comparison... in an older edition.
In v3.5 of Dungeons & Dragons, Wizards had the option to something like what you suggest. They could opt to prepare only part of their spells and keep the rest open, so they could be filled in later. Wizards were considered one of the most ridiculously powerful and flexible classes in the game.
What you are proposing is even more powerful, because the Wizard at least required at least 15 minutes of rest to prepare a new spell in an open slot.
In my games, I've seen this feature used by a few players. (Most opted to just prep everything at the start of the day.) Here's what happened:
Gameplay slowed down. Whenever my players would run into a problem that they could conceivably solve half an hour later, the Wizard would go down his spellbook and look for possible solutions that they could prep. Wizards tend to have a lot of spells in there, so this would take a while. Lots of banter would ensue over which spell the Wizard might prep. Half the time, in the end no spell was prepped and a different solution was chosen altogether.
Players would look to the Wizard even more than normal. A Wizard who preps everything at the start of the day is incredibly flexible. A Wizard who can prep anything once you know what you're up against is just unreasonably flexible.
The Wizard would ultimately stop using the feature because it's a huge mental load. As the spellbook grows, the number of things a Wizard could do in a given situations rises very rapidly. It was doable for a few levels, but it quickly became too much.
In the end, most players chose not to use the ability because it was both too good and too cumbersome. It would take too much table-time away from having fun and made a lot of encounters too easy.
You are likely to experience the same things as level goes up; the caster's options will go up rapidly, which increases their cognitive load and makes them much more powerful. If you think picking what to do in combat is bad when you have 5 spells to pick from, imagine how bad it gets when you have your entire spell list to chose from.
One of the reasons 5e cut down on how many spells you can cast at a given moment is because combat flows a bit more smoothly that way. And, like I said in my comment, there is a reward in actually thinking ahead now that you will lose. It's not your fault (or problem) that your players aren't gathering information about what they will be facing.
(As for hounding players to select spells; I always rule that after a long rest you prep the same spells as the day before unless you tell me you want to make changes. Most players have a default loadout that they use most of the time anyway).